The desire to have broadband information services available at residential customer premises has promoted intense interest in the use of optical fiber media. Fiber-to-the-home, in which optical fiber transport is used over the entire path, is appealing because of the large information capacity of optical fiber. Various techniques for communicating over optical fiber include time division multiplexing (TDM) techniques, wavelength division multiplexing (WDM) techniques, sub-carrier multiplexing (SCM) techniques, and spatial division multiplexing (SCM) techniques.
Passive Optical Networks (PONs) are used to provide for communication of two-way high-bandwidth data between a system head end and various end nodes. In PONs, there may be no intervening active components between the system head end or central office and a customer's premises. In other words, PONs generally require no active components for directing optical signals between the system head end and the remotely located end nodes at the customer's premises. PONs therefore, require no power or electronic processing in the field to direct optically encoded information to its destination. A conventional PON generally includes a fiber-optic star or tree coupling device which connects a plurality of optical paths extending from the system head end to the remotely located end nodes. Downstream optical signals are transmitted from the system head end over an optical distribution fiber to an outside plant node where the signal is passively split and distributed to the remotely located r end nodes. The end nodes may transmit optically encoded signals upstream to the outside plant node to form a multiplexed signal on the distribution fiber for distribution to the system head end.
For downstream transmission from a system head end to remotely located end nodes, PONs may implement time division multiplex (TDM) techniques, wavelength division multiplex (WDM) techniques, or other techniques for partitioning data destined for individual end nodes. For upstream transmissions, where many end nodes may access the fiber media, the multiple access may be achieved by, for example, time division multiple access (TDMA), wavelength division multiple access (WDMA), or code division multiple access (CDMA), or combination thereof.
One of the most basic schemes utilizes TDM for downstream data transmissions and TDMA for upstream data transmissions and is often referred to as a power-splitting TDMA PON. Downstream data is a “broadcast and select” time-division multiplexed (TDM) stream of data frames. Each end node receives a broadcast copy of the downstream TDM data and selects its own specific data based on an address within the TDM stream. This TDM stream may occupy a single wavelength. For upstream data transmissions, end nodes achieve multiple access by synchronizing their upstream transmissions so that they occur in a pre-assigned interval. This synchronization reduces or eliminates overlap of upstream data transmissions from the splitter/combiner device after multiple data streams from end nodes are combined. The upstream may be transmitted on a single wavelength and generally is a distinct wavelength from the downstream.
Given the wide variety of potential broadband communication services to be supported over a shared communication network, it is desirable to provide an efficient digital data transmission protocol for the upstream communication paths. Such a data transmission protocol should most fully utilize the available upstream channel bandwidth. Although the art of transmitting downstream data from a system head end to remotely located end nodes is well developed, there remain some problems with the transmission of the upstream data. For example, upstream data communication may implement a time division multiple access (TDMA) technique that includes synchronized multi-step request/grant negotiations between the system head end and the ONUs at the remotely located end nodes in order to prevent upstream packet collisions and pack the upstream transmissions closely together. This level of complexity is undesirable and inefficient and consumes significant network resources.
A less complex upstream data transmission technique employs a “dumb” master/slave protocol with fixed time slot assignments for end nodes to transmit upstream data. The time slots may have fixed sizes or may have cyclically varied fixed sizes. This technique is highly inefficient because the system head end commands all end nodes to halt upstream transmissions and specifies a single end node for transmission. The end node is permitted to transmit after sufficient delay from receiving the halt command since the prior transmitting end node may still have a frame of upstream data in transit that it may collide with. However, when there is no explicit size reservation per upstream data transmission, the allocated time slots must be large enough to accommodate at least one maximum length data frame even when an end node only needs to transmit a minimum length frame or no frame at all since time slots are allocated without regard for need.
Thus, there is a general need for a less complex and efficient way of transmitting upstream data in a PON. There is also a need for a method and system that efficiently transmits upstream data in a PON. There is also a need for a method and system for coordinating transmission of upstream data between remotely located end nodes and a system head end in a PON. There is also a need for a method and apparatus that eliminates the transit delay wait periods that result from long distribution fibers when coordinating transmission of upstream data in a PON. There is also need for a method and apparatus that allow variable length upstream data frames to be sent by each end node without explicit size reservation message exchange by the system head end. There is also a general need for a method and end point-directed multiple access PON with centralized scheduling.
The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner.